Multipactor phase control



Feb. 23 31937 P. T. FARNSWORTH MULTIPACTOR PHASE CONTROL Filed May 7, 1935 V IN VENTOR PHIL 0 7T FARNSWOR TH.

Patented Feb. 23, 1937 UNITED STATES MULTIPACTOR PHASE CONTROL Philo T. Farnsworth, San Francisco, Calii'., as-

signor to Farnsworth Television Incorporated, San Francisco, Calif., a corporation of California Application May 7, 1935, Serial No. 20,157

19 Claims.

My invention relates to phase control in the electron multipliers or multipactors, as I prefer to call them, and more particularly to a means and method whereby electrons component of a multiplied stream may be segregated to produce a current and a potential differing in phase from those normally present therein.

The word multipactor as used in the following specification and claims is a name selected by me for an electron multiplier wherein electrons repeatedly bombard a surface at a velocity sufiicient to produce secondary electrons therefrom at a ratio greater than unity so that an electron multiplication takes place. The term J is deemed inclusive of straight multipliers, and

those devices of like action which are sufiioiently sensitive to self-oscillate. Such devices and methods of operating them have been described in detail in my prior applications as follows: Electron multiplier, 692,585, October 7, 1933;

Oscillation generator, 733,837, July 5, 1934; and

Methods of electron multiplication, 706,965,

January 17, 1934.

The present invention has, among other objects, the following: To provide a more efficient electron multipactor; to provide a more efficient method of cathode energization in a multipactor; to provide a means and method for obtaining multipactor cathode currents 180 out of phase with'the multipactor cathode potentials; to provide a multipactor wherein cathode currents exist both 90 and 180 out of phase with the cathode potentials; to provide a means and method for utilizing apertured cathodes in a multipactor; to provide a means and method for collecting electrons passing through apertured multipactor cathode; to provide a simple and eflicient multipactor tube and circuit; and to provide a means and method of increasing multipactor cathode potentials.

My invention possesses numerous other objects and features of advantage, some of which, together with the foregoing, will be set forth in the following description of specific apparatus embodying and utilizing my novel method. It is therefore to be understood that my method is applicable to other apparatus, and that I do not limit myself, in any way, to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, within the scope of the appended claims.

Referring to the drawing:

Figure 1 is a longitudinal view partly in sec. tion and partly in elevation of a preferred embodiment of a multipactor utilizing my present invention.

Figure 2 is a cross sectional view taken through the electrodes of the tube of Figure 1, together with a diagrammatic work circuit attached thereto.

Figure 3 is a graph showing the phase relationship of the cathode currents in the ordinary electron multipactor.

Figure 4 is a graph showing the phase relationship of a cathode current and potential oh- 5 tained in the multipactor or the present invention.

The simple multipactor previously described by me comprises broadly a device having a pair of opposed secondary emissive surfaces. In this regard it should be pointed out that any conductive surface will become secondarily emissive provided electron velocities of impact are sufiiciently high. A collecting anode is positioned within the space bounded by the opposed cathodes. If an alternating potential be applied to the opposed cathodes and a direct potential placed upon the collector or anode, either straight" multiplication or multiplication suificient to sustain self-oscillation will occur, due to the fact that as electrons are oscillated back and forth Within the space between the two cathodes secondaries will be given off at each impact until an equilibrium state is reached. When used as a multiplier, if initial electrons are introduced into the space between the two cathodes, the output current as shown in the anode circuit will increase to the point where limiting factors intervene and will be proportional to the number of electrons introduced into the space. The multiplication will also depend upon the number of impacts which in turn will depend upon the excitation of the electrodes. The cathodes may be excited, for example, wholly by an incoming signal, in which case the number of impacts will differ for differing cathode potentials, or by a steady frequency applied from an exterior source. When used as an oscillation generator the multipactor will supply its own cathode potentials.

I prefer to operate the device so that the time of flight, as determine-d by the distance between the cathodes and the voltages applied to the electrodes, is the same as the oscillation time of the cathode frequency and in this condition the device is most sensitive, but I do not Wish to be limited to this mode of operation as there are several other modes equally useful.

I also prefer to operate the multipactor with either electrostatic or magnetic guiding fields to prevent immediate collection of the electrons and to allow certain number of electron impacts to \take place before collection, thus raising the multiplication. Other uses and modifications of my broad invention may be obtained from the cases previously referred to.

' In these prior multipactors, the current fiowing out of the cathodes is 90 degrees out of phase with the cathode potential. 'lE'he present invention broadly comprises utilizin g perforated cathodes with a means positioned back of the cath-- e0 which is then supplied flowing into the cathode.

out of phase with the potentials so as to produce an electromotive force in phase with the cathode excitation potential thus raising the efliciency of the device and making the device more completely useful inasmuch as the regular cathode current will be present also.

Broadly, in terms of structure, my invention comprises a pair of opposed apertured cathodes preferably in the form of screens having an anode therebetween, each cathode being. backed by a non-secondarily emissive collecting plate, the collectors being spaced a suflicient distance back of the apertured cathode so that there will be a time lapse between the time an electron passes through the perforated cathode until it is collooted by the plate. If, then, this time be regulated and the collector connected to the cathode, a current will result from the collected electrons which flows into the cathode outof phase with the potential thereby producing an electromotive force in phase with the potential thereon and reinforcing it.

While I have shown my preferred embodiment in the form of an oscillator, utilizing for a start electrons always existing in the space between the cathodes, or electrons emitted from the cathode at room temperature, photoelectrically or thermally, I wish it to be distinctly understood that the present invention may also be usedas a straight current multiplier and electrons introduced into the space between the cathodes from an exterior source and thereafter multiplied. Such electrons may be introduced through an aperture in either of the cathodes or in any other manner which will be easily understood by those skilled in the art. Another example of this particular use would be in conjunction with a cathode ray image dissector tube such as described by me in my Patent No. 1,773,980 for Television system, issued August 26, 1930, the multipactor structure being positioned so that electrons passing through the scanning aperture will pass into the space between the two cathodes.

Other broad aspects of my invention will be apparent by consideration of a detailed description of a preferred embodiment of my present invention as shown in Figure 1. As above explained, this tube is intended to operate as a self-oscillator and the various electrodes are enclosed in an envelope I. As before pointed out, if this device is to be used as a straight multiplier, the envelope I may well be a portion of an envelope of another thermionic apparatus having therein a stream of electrons it is desired to multiply.

A pair of apertured cathodes 2-2 are positioned to describe a portion of a cylindrical surface and thus will have their concave surfaces opposing each other. For convenience in operational descriptions the cathodes have been labeled A and B. While it is not necessary nor always desirable to make the cathodes of this shape, I prefer to do so because a static field is provided between the two cathodes which tends to prevent immediate electron collection by a central anode 3 which may either be in the form of a single upright wire or, as shown here, a wire provided with spiral open mesh grid 4 which increases the chances of collection by the anode, but does not cause great obstruction to the stream. Other anode structures may be optionally used without departing from the spirit of my invention.

While almost any materials will emit secondary electrons when bombarded with electrons travelilar substances thereon following rather closely the general procedure for the construction of -photoelectric surfaces as is well known in the art. I have found that caesium on silver oxide is highly satisfactory both as a photoelectric surface and as a secondarily emissive surface, and I therefore prefer to form the screens 2 of solid silver, oxidize them and admit caesium vapor into the tube in order to sensitize the silver oxide. Other methods of sensitization may be employed equally as well or straight unsensitize'd surfaces of various metals may also be employed.

The maximum number of secondary electrons emitted by the surfaces when properly impacted will of course depend in part upon the nature of the material, nickel, for example, will emit 1.64 electrons per primary and caesium from 4 to 6 electrons per primary when traveling at proper velocities. In order to make the tube as sensitive as possible and to make it capable of self-oscillation I prefer to use caesium in order that the maximum of secondaries be obtained.

Immediately back of each apertured cathode I prefer to mount a pair of solid collecting electrodes 55 to which the screens are attached by jumpers 6. I prefer that the screens 2 and collecting electrodes 5 shall be concentric and I also prefer to make the curvature of the anode grid 4 to be concentric with the screen and collectingv electrode. The collecting electrodes are supported in the envelope by longitudinal reenforcement strips 1 formed to have a longitudinal rib 9 to which leads I0 are welded. Each lead I0 is sealed through side arms II and the anode structure 3-4 may conveniently be supported by anode lead I2 passing through the usual type of end stem I 4. In this way, a rigid assembly is obtained and one wherein there is maximum surface distance between the three leads of the tube, thus making the tube adaptable for high frequency use.

Collecting electrodes 5 may be of nickel, tantalum, tungsten, molybdenum, but I find that nickel is very satisfactory. I, however, prefer to make the collecting electrodes 5 non-secondary emitting, and if there is too much secondary emission from any of the particular metals chosen, they may be made non-emitting as far as secondaries are concerned, by carbonizing them. In my preferred embodiment, I prefer to use carbonized nickel for these collecting electrodes, thus insuring that no secondary emission will take place therefrom.

I prefer to describe the multipactor as hooked up as a self-oscillator in the simplest form of circuit, such as shown in Figure 2. Here, the two cathodes are attached by wires I5I5 to opposite ends of a tuned circuit comprising a variable condenser IB and an inductance I! which latter is provided with a center-tap I8, connected to ground I9. The anode is supplied with a steady potential from anode source 20 through wire 2|, the other end of source 20 being grounded. Upon energizing the anode, electrons within the space between the two cathodes 2-2, or electrons emitted from the screens or cathodes due to thermal or photoelectric emission at room temperature will be accelerated toward the anode, some of them will pass through the anode, and will impact the opposite cathode with sufficientvelocity to emit secondaries.

If we consider, for purposes of illustration, that the cathodes are not apertured, secondaries will be emitted by electrons impacting the opposite cathode and the secondaries will be reac celerated toward the anode and toward the first cathode, until an oscillating electron stream will be quickly built up and both a potential and current will be present across the oscillating circuit l6-l1 and therefore the cathodes. If the cathodes are sufficiently sensitive the device will immediately stabilize itself in self-oscillation. Load connections may be applied to the oscillating circuit l6--Il and useful currenttaken therefrom.

When the multipactor is oscillating, still considering that the cathodes are solid and not apertured, the current-potential phase relationships on the cathodes are shown in Figure 3. vPositive potential half-cycle 22 occurs on cathode A and negative potential half-cycle 23 denotes the potential on the opposite cathode B, and so on. The current half-cycle 2! is the current flowing out of cathodeA and the current halfcycle 25 is the current flowing out of cathode B. It will be seen that the current is out of phase with the potential by 90 degrees.

Returning now to the use of apertured cathodes, it will be found that with steady stated conditions, a certain number of electrons, the actual number depending upon the ratio of surface to space in the cathode screens, will be intercepted by the screens and as to that portion which each time is intercepted, the current-potential relationships will remain the same as shown in Figure 3. However, inasmuch as the cathodes are apertured, each time the electron cloud reaches the plane of the cathodes, a certain number will not be intercepted but will pass through the cathodes and continue on toward the collecting electrodes to be collected thereby without the emission of any secondaries upon impact. There is, therefore, a current component developed due to this collection back of the cathodes which is different in time-phase from that flowing out of the cathodes themselves, due to the fact that it takes a definite time for the electron to pass through the space between the screen and the collecting electrode.

It is, therefore, relatively easy to adjust the space, or with a given space to adjust the time of flight so that the current which flows into each collecting electrode is exactly out of phase with the potential applied to the cathodes. This produces an electromotive force which is in phase with the energization potentials inasmuch as the collecting plate is connected directly to the cathode.

A graph, therefore, which illustrates the relationships of the current and potential purely as regards the collecting electrode is shown in Figure 4. Here, half-cycle 26 represents the energization potential of cathode A, and half-cycle 21 represents the current flow into the opposite cathode B. Half-cycle 28 represents the potentiol of the cathode B and current half-cycle 2.9

represents the current flowing into cathode A by the proper spacing of an apertured secondary emissive cathode and a backing electrode which is relatively non-emissive. This spacing between the two may be varied to obtain phase relationships which may be desired for various and sundry purposes and uses within or without the multipactor structure.

I claim:

1. The method of electron multiplication by electron bombardment of a pair of apertured electrodes which comprises oscillating electrons between said electrodes at a velocity sufficient to create secondaries at a ratio greater than unity upon impact therewith, and collecting electrons passing through the apertures in said electrodes.

' 2. The method of electron multiplication by electron bombardment of a pair of apertured electrodes which'comprises oscillating electrons between said electrodes at a velocity sufficient to create secondaries at a ratio greater than unity upon impact therewith, collecting electrons passing through the apertures in said electrodes, and utilizing the collected electrons to produce a potential in phase with the potential of said alternations.

3. The method of electron multiplication by electron bombardment of an apertured surface which comprises bombarding said surface with electrons at a velocity suflicient to create secondaries at a ratio greater than unity upon impact therewith, collecting electrons passing through said surface, and utilizing further secondaries created by the interception of electrons by said surface to create more secondaries.

4. The method of electron multiplication by electron bombardment of an apertured surface which comprises bombarding said surface with electrons at a velocity sufficient to create secondaries at a ratio greater than unity upon impact therewith, collecting electrons passing through said surface, and utilizing the latter secondaries as primary electrons to repeat the cycle.

5. The method of electron multiplication by electron bombardment of a pair of apertured electrodes which comprises creating a stream of electrons between said electrodes, alternately energizing said electrodes to cause electron im pact with the solid portions thereof at sufiicient velocity to create secondaries at a ratio greater than unity, adding the secondaries thus produced to said stream, and collecting the electrons passing through said electrodes.

6. The method of electron multiplication by electron bombardment of a pair of apertured electrodes which comprises creating a stream of electrons between said electrodes, alternately energizing said electrodes to cause electron impact with the solid portions thereof at suflicient velocity to create secondaries at a ratio greater than unity, adding the secondaries thus produced to said stream, collecting the electrons passing through said electrodes, and utilizing the collected electrons to produce a potential in phase with the potential of energization.

7. A multipactor comprising a pair of opposed apertured surfaces adapted to produce secondaries on electron impact therewith, means for energizing said surfaces with an alternating potential to cause repeated oscillation of electrons therebetween thereby creating a space current continuously augmented by secondaries liberated from said surfaces, and means for collecting electrons passing through said surfaces.

8. A multipactor comprising a pair of opposed apertured surfaces adapted to produce secondaries on electron impact therewith, means for energizing said surfaces with an alternating potential to cause repeated oscillation of electrons therebetween thereby creating a space current continuously augmented by secondaries liberated from said surfaces, and means out of said space current for collecting electrons passing through said surfaces.

9. A multipactor comprising a pair of opposed apertured surfaces adapted to produce secondaries on electron impact therewith, means for energizing said surfaces with an alternating potential to cause repeated oscillation of electrons therebetween thereby creating a space current continuously augmented by secondaries liberated from said surfaces, and means directly connected to each of said surfaces for collecting electrons passing through said surfaces.

10. A multipactor comprising a pair of opposed apertured surfaces adapted to produce secondaries on electron impact therewith, means for energizing said surfaces with an alternating potential to cause repeated oscillation of electrons therebetween thereby creating a space current continuously augmented by secondaries liberated from said surfaces, and means backing said surfaces for collecting electrons passing through said surfaces. Y

11. A multipactor comprising a pair of opposed apertured surfaces adapted to produce secondaries on electron impact therewith, means for energizing said surfaces with an alternating potential to cause repeated oscillation of electrons therebetween thereby creating a space current continuously augmented by secondaries liberated from said surfaces, and means back of each of said surfaces for collecting electrons passing through said surfaces and connected to the respective surfaces, the said latter means being positioned at such a distance behind the respective surfaces that the potential produced by electron collection is in phase with the potential of surface energization.

12. The method of exciting electron emission from an apertured cathode which comprises withdrawing from said cathode the electrons emitted thereby due to photoelectric and thermal emission at room temperature, reversing the direction of said electrons and accelerating them against said cathode with sufiicient velocity to cause the emission of additional electrons by impact thereon, repeating said operation upon all of the electrons emitted thereby to build up the emission, collecting thatportion of the electrons passing through the apertures in said cathode and utilizing the potential produced by the collected electrons to increase the acceleration of the multiplied electrons.

13. The method of generating an electronic space charge between a pair of apertured electrodes which comprises applying a high frequency alternating potential between said electrodes, said potential being adapted in magnitude and period to cause electrons in the space between said electrodes to impact one of said electrodes with a sufficient velocity to cause secondary emission therefrom and to cause the emitted electrons to impact the other electrode at a like velocity to cause secondary emission, repeating the cycle with electrons intercepted by said cathode, collecting electrons passing through said electrodes. utilizing the collected electrons to produce a potential, and adding the latter potential to the first-mentioned potential.

14. The method of generating an electronic space charge between a pair of apertured electrodes which comprises applying a high frequency alternating potential between said electrodes, said potential being adapted in magnitude and period to cause electrons in the space between said electrodes to impact one of said electrodes with a sufllcient velocity to cause secondary emission therefrom and to cause the emitted electrons to impact the other electrode at a like velocity to cause secondary emission, repeating the cycle with electrons intercepted by said cathode, collecting electrons passing through said electrodes, utilizing the collected electrons to produce a potential, and adding the latter potential to the first-mentioned potential and in phase therewith.

15. The method of maintaining a substantial- 1y pure electronic discharge between a pair of apertured electrodes which comprises introducing a cloud of electrons therebetween and cyclically varying the potential between said electrodes to cause electrons component of said cloud to impact one of said electrodes with sufilcient velocity to cause secondary emission therefrom at a ratio greater than unity and to withdraw the emitted electrons into said cloud, collecting electrons passing through the apertures in said electrodes and utilizing the potential produced by the collection thereof to re-enforce the varying potential between the electrodes.

16. A multipactor comprising an apertured surface capable of producing secondary electrons upon electron impact therewith, means for energizing said surface with an alternating potential to cause repeated oscillation of electrons against and away from said surface thereby creating an electron cloud augmented at each impact, and means for collecting. electrons passing through said surface.

17. The method of electron multiplication which comprises cyclically bombarding a material capable of emitting secondary electrons at a ratio greater than unity to produce an oscillating cloud of electrons augmented at each bombardment cyclically withdrawing a predetermined number of electrons from said cloud, and separately collecting the withdrawn electrons.

18. The method of electron multiplication which comprises cyclically bombarding a material capable of emitting secondary electrons at a ratio greater than unity to produce an oscillating cloud of electrons augmented at each bombardment cyclically withdrawing a predetermined number of electrons from said cloud, separately collecting the withdrawn electrons, and utilizing the energy collected to increase the velocity of electrons in said cloud.

19. The method of electron multiplication which comprises creating an oscillating electron cloud having a mixed phase relation, cyclically augmenting said cloud by secondary emission caused by electron impact in phase with the oscillations of said cloud, and segregating at least a portion of the electrons out of phase with said inpacts.

' PHILO T. FARNSWORTH. 

